The present invention relates to a new and improved method of, and apparatus for, signal recognition in a remote control system, wherein on the transmission-side or transmission-end of the system, the remote control signal is produced by a desired or intentional influencing of the voltage course during an influencing period or time span in a region about at least one zero point transition or zero cross-over of the transmitter-end alternating-current mains voltage, such is impressed and transmitted via a power supply network and, at the reception-side or receiver-end of the system, the appearance of such a remote control signal is accomplished by comparing the receiver-end alternating-current mains voltage, during certain comparison periods or time spans, at the region of receiver-end alternating-current mains voltage zero point transitions or cross-overs. A remote control system of this type is known, for example, by the name of CYCLOCONTROL; see for example the article "Use of London's Electricity Supply System for Centralised Control; by A. J. Baggot, B. E. Eyre, G. Fielding, F. M. Gray; PROCEEDING IEE, Vol 125 No. Apr. 4 1978, pages 311 to 327", particularly paragraphs 5.2 to 5.3.4.
According to the prior art, in remote control systems of the type here under consideration, in order to produce a remote control signal on the transmission-side or transmitter-end, the course of the mains voltage in the region of zero point transition of the, for example, 50 Hz alternating-current voltage curve, is temporarily changed such that on each phase of the secondary side of a medium voltage transformer, a short-circuit load is produced by temporarily switching-on a thyristor for each phase. This pulsatory or surge-like load takes place, for example, during an influencing period or time-span ranging from minus to plus 25 electric degrees about at least one zero point transition or zero cross-over of the alternating-current mains voltage curve.
As a result of this pulsatory load in the medium voltage transformer, there is produced an influence on the alternating-current mains voltage curve. This influence, for example, appearing as a deviation from the sinusoidal form, constitutes the remote control signal. The remote control signal is impressed, as explained above, upon the alternating-current mains voltage and is transmitted in the connected and also superimposed power supply network.
At this power supply network, for example via other transformers, receivers connected at the erection site of remote controlled energy consumers or loads, evaluate the mains voltage which has been distorted by the mentioned influence i.e. they respond to thus produced remote control signal.
Heretofore known receiving methods and receivers are based upon monitoring, for example integrating the receiver-side course of the mains voltage in each case from a zero point transition of the receiver-side mains voltage during a time span from half the value of the influencing period and comparing the integration result with integration values which are previously obtained in the same manner. The integration results are normally different, depending on whether at an evaluated zero point transition at the transmitter-end, there has or has not been an influence of the type described.
Therefore, the presence of a remote control signal is recognised at the receiver-end from the differing integration results and this signal may be further evaluated in a known manner, for example in a decoding device. In this connection, see, for example, the above-quoted article.
Remote control systems are known in which in order to transmit a piece of information, the temporal sequence of the zero transition of the mains voltage, is temporarily changed. In another known system, in order to mark a signal the value of the mains voltage is influenced at a certain point in the cycle, e.g. shortly before until shortly after the zero cross-over, for instance during the transition from a positive to a negative voltage half-wave. This last-mentioned process has, in particular, the advantage that there can be used simpler influencing means, or, expressed in other words, has the advantage of particularly simple transmitters. For a three-phase network or mains, they may essentially consist of three thyristors connected from each one of the phases to the neutral conductor. These thyristors are switched-on shortly before the zero point transition or cross-over of the voltage from the positive to the negative half-wave. The thyristor firing leads to a breakdown of the voltage at the relevant phase and to the formation of a short-circuit current. This short-circuit current which, in practise, is mostly inductive, again reaches the value of zero relatively shortly after the zero point transition of the supplying voltage, whereupon the thyristor automatically is extinguished and the voltage jumps back to the uninfluenced value. The signal consisting of the change in voltage in the region of this zero point transition is propagated over the entire mains. The described process, in particular, has an effect upon the supplying superimposed mains, so that the signal may also be received in neighbouring mains regions. The receivers for a transmission system of this type are constructed, for example, in such a manner that they integrate for a brief period of time the mains voltage after each zero point transition from the positive to the negative voltage half-wave which is triggered by the zero point transition itself, and they store the result until the next zero point transition from the positive to the negative half-wave. If the voltage curve remains uninfluenced, i.e. if no remote control signal is produced on the transmission side of the system, then the renewed integration practically reproduces the value which was previously obtained. However, if the alternating-current mains voltage was influenced, i.e. reduced on the transmission side or transmitter-end of the system in the previously described manner, then the integration now produces a noticeably lower value. This lower value is interpreted by the receiver as a remote control signal, for example by difference formation or subtraction.
Remote control receivers of this type have the serious disadvantage that, under certain circumstances, they repeatedly miss remote control signals. This is so because between the mains voltage at the transmission site and the mains voltage at the receiver site, governed by the presently prevailing mains configuration and mains load situation, considerable phase differences may exist, so that the transmission time derived from the mains voltage curve at the transmission site possibly no longer agrees, in the required manner, with the evaluation time also derived from the mains voltage curve at the receiver site. The signal pulse constituted by the described voltage reduction at the transmission site also not only experiences an attenutation or damping, but also a distortion by the lines, transformers, capacitors and the loads which are present in the mains. Particularly, where there is a small mains load, transients may occur, especially with relatively short evaluation times, which, although a remote control signal is transmitted, i.e. reducing the voltage at the transmission site in the region of a zero point transition, may lead to a result being produced at the receiver-end during the evaluation period in the receiver which is the same as for a transmission-site uninfluenced voltage curve.